The present invention relates to power-switching semiconductors and, more particularly, to a novel method and circuitry for the controlled switching of non-regenerative power semiconductor devices to substantially reduce EMI while providing acceptable "active region" dissipation in the devices.
It is well known to minimize the time that a non-regenerative power-switching semiconductor device spends in the "active region", to prevent excessive power dissipation in that device. Hitherto, the need for reducing excessive switching power dissipation has been met by switching the device from a fully-on condition to a fully-off condition in as rapid a manner as possible, consistent with the maximum dV/dt or dI/dt limits of the device. The relatively rapid voltage and/or current change in the device has generated considerable amounts of electromagnetic interference (EMI).
If all power-switching semiconductors of the same type had identical characteristics, it might be possible to turn the device on or off by utilizing a programmed drive source having a desired rate of change. In practice, however, the tolerance on the semiconductor control element, e.g. on the gate voltage threshold in a power field-effect transistor (FET) or an insulated-gate rectifier (IGR), is often greater than the variation of the signal required at that control element for a transition from a substantially turned-off condition to a substantially turned-on condition, or vice-versa. Thus, one commonly used control electrode driving method is to change the charge in a control electrode capacitance (such as the internal gate electrode capacitance in an FET or an IGR, often having an external fixed capacitance in parallel therewith to provide a total capacitance) from a current source. This method may result in a control electrode (gate) voltage characteristic which approximates desired characteristics only over a portion of the switching time interval, due to the drain-gate or anode-gate capacitance, commonly known as the "Miller-capacitance". As the device starts to turn on or off, the Miller capacitance couples the drain, or anode, voltage change into the gate circuit and slows the rate-of-change of the gate voltage. If the interelectrode capacitances of all the devices of the same type were exactly the same, this might be a useable approach. However, since all devices are not identical, destructive switching effects may occur. It has been further observed that when the control (gate) element is driven by a high impedance source, such as a current source, a number of load conditions are possible which will result in destructive on/off self-switching of the power-switching device.
Accordingly, it is desirable to provide a control element drive signal which will cause the device to turn on and/or turn off slowly enough to substantially eliminate EMI, while maintaining the device switching losses at an acceptable level and to do so without introducing self-destructive oscillations in the power device.